This invention pertains generally to fasteners and particularly to a thermally compensating insert fastener for interconnecting two sections of a missile.
It should be appreciated in an apparatus such as a missile it is often desirable to construct a missile in sections. Typically the missile includes a radome, a guidance section, a warhead section, a flight control section and a rocket motor section. In a missile, the guidance section can be pressurized to prevent high voltage arcing during operation at high altitudes. Numerous seals are employed to maintain guidance section pressure during passive captive carry flight and during active operational free flight. Loss of pressure during flight can cause the guidance section which houses a target seeker to operate incorrectly.
One known cause of loss of pressure in the guidance section during flight is from an improper seal at an interconnection of two sections of the missile. One type of interconnection for connecting a forward titanium shell with an aft titanium shell includes an inner titanium support structure which is mutually interconnected with the forward and aft titanium shells with a set of eleven fasteners. Each of the fasteners include an external countersunk screw and a "Keensert", which is an externally threaded bushing having an internally mounted nut, affixed within the internal support structure.
With such an arrangement, when the fasteners are tightened, deformation of the forward and aft shells will occur as the forward and aft shells are drawn toward the internal support structure. Since the inner diameter of the forward shell will not exactly match the outer diameter of the aft shell and the inner diameter of the aft shell will not exactly match the outer diameter of the internal support structure due to manufacturing tolerances, varying radial gaps will exist. As the fasteners are tightened, scalloping of the forward and aft shells will occur. Typically, visible measurable shell scalloping between fastener locations can be observed with the forward shell being the most deformed. The interconnection also includes two seals using butyl O-rings. One O-ring seals the forward shell to the support structure. The other O-ring seals the aft shell to the support structure. When the forward shell and the aft shell are interconnected with the support structure, the seals are uniformly compressed. As the fasteners are tightened, mechanical deformation is induced and the seals are no longer uniformly compressed. Cold temperatures inhibit the seal response to changes in compression which occur during rapid airframe heating. The latter can induce seal failure and pressure loss during flight.